1. Field of the Invention
The field of the invention relates to an electronic coder, decoder (CODEC) device for converting analog audio signals to digital and digital audio signals to analog. The field of the invention further relates to a stereo audio CODEC with data format conversion, analog and digital filtering, and analog mixing capabilities.
2. Brief Description of the Related Technology
Audio CODEC devices are known in the art. Many of these devices have 8-bit data ports. Two examples are: the AD1848 SoundPort Stereo CODEC, manufactured by Analog Devices of Norwood, Mass.; and the CS4231 Multimedia Audio CODEC, manufactured by Crystal Semiconductor Corporation of Austin, Tex. These CODEC devices provide 16-bit audio data over an 8-bit parallel interface between the CODEC and an external industry standard architecture (ISA) has or an extended industry standard architecture (EISA) bus. These CODEC devices also provide analog-to-digital (ADC) and digital-to-analog (DAC) conversions of stereo audio signals, and certain mixing functions.
The CS4231 device includes a 16-sample on-chip first-in first-out (FIFO) memory for holding signal samples for the record or playback paths, but does not provide other larger off-chip local memory for storing additional samples. Thus, when either the record or playback FIFO needs to be read from or written to, a direct memory access (DMA) or programmed input/output (I/O) data transfer operation must occur with off-chip system memory. Since each CS4231 FIFO, for record or playback, may hold only 16 samples, a DMA or PIO request for a write to, or read from, the respective FIFO must occur before the respective FIFO is full (during record) or empty (during playback) to prevent data loss and sound discontinuities. No capability exists with the Analog Devices or Crystal CODEC devices to retrieve data from, or send data to, an off-chip local memory to avoid a DMA or PIO cycle on the external system bus during record or playback modes.
Additionally, the CS4321 device and other current art devices do not provide programmable thresholds on its on-chip FIFOs. Additionally, the CS4231 device provides only 16 mA bus drivers. If 24 mA drive current is required, additional off-chip drivers must be installed.
The AD1848 device does not contain an on-chip FIFO or other memory for storing samples along the playback or record path. All playback path data is sent from off-chip external system memory through the device 8-bit parallel port interface to the CODEC device. Likewise, recorded data is sent from the CODEC device through the 8-bit parallel interface to be stored in external system memory. DMA and/or PIO cycles are required by the CODEC during playback or record mode in order to transfer audio data. Additionally, the AD1848 device requires the use of an external 74.sub.-- 245 transceiver to interface with the ISA bus.
Both the CS4321 and AD1848 devices provide only 14 sample rates for record and playback modes, which range between 5.51 and 48.0 Khz. Also, each device can only use the same sample rate for simultaneous record and playback. Record and playback rates are not independently programmable.
Neither the CS4321 nor the AD1848 device contain an external serial audio data port or selectable bus drive capability. Additionally, neither current art CODEC device contains a CD-ROM interface, or serial EPROM interface for Plug-n-Play support. Also, these devices do not provide an analog feedback loop or an analog audio synthesizer input in the mixer section of the CODEC.